In the research work of silicon devices, it is often needed to measure and control accurately the thicknesses of SiO2 thin films. Although it is well known that SiO2 thin films exhibit vivid colours under sunlight due to interference effects, yet there is no simple relationship between the thickness of the thin film and the wave length of the light corresponding to the observed colour. Described in this paper is a method for the measurement of the thicknesses of thermally grown SiO2 thin films. From the measured data, we have calculated the index of reflection of the SiO2 thin film as well as the phase shift difference between the SiO2-Si interface and the Si-air interface.By measuring the thicknesses of a set of SiO2 thin films of different colours, a chart is obtained which lists the thicknesses and the corresponding colours of the thermally grown SiO2 thin films. The results are found to be in fairly good agreement with that calculated from Roller's table. Our results include the thickness range of 3300-4200? which was not covered in Rollet's table.